How are alleles and traits related?

The alleles that a person has determine what traits that person will have.

Alleles are different versions (DNA sequences) for the same gene.

For example, the same person can have two different alleles for the gene that controls eye color (Brown or blue eyes).

This person would be heterozygous (Bb) for the eye color "trait" because they have two different "alleles" at that gene location. But they would only show the dominate color (B).

Here you see brown eyes (a trait) but with two different genes: BB and Bb. And also you can see the color for blue (bb), another trait.

Many times the words gene, allele and trait (what is seen) are used interchangeably but in reality they are referring to levels of specificity.

A trait (phenotype) is determined by genes (region of a chromosome) which can have different alleles (versions) on homologous chromosomes.

The different alleles are called genotypes, or what is in the genes. What is expressed, or seen, is called the phenotype.